1. Field of the Invention
The present invention relates to an apparatus for tightly covering the surface of a sheet, such as a label or identification document, with a thermoplastic film by heat-laminating the film to the document.
2. Description of the Prior Art
Identification documents essentially comprise a card or sheet material containing information relating to the bearer. Generally, a portion of the information is in the form of a photograph. Identification cards (I.D. cards) are used e.g. to establish a person's authorization to conduct certain activities (driver's licence) or the authorization to have access to certain areas (employee I.D. cards) or to engage in credit transactions (I.D. credit cards).
In view of the widespread use of I.D. cards, especially in commercial transactions, such as cashing checks, credit purchases, etc., it is important that the information contained in the I.D. card is protected against mechanical and/or chemical damage and that the I.D. card gives maximum protection against counterfeiting.
Normally the information in the I.D. card is protected by lamination between plastic sheets serving as support and covering sheet.
Many attempts have been made to obtain a seal that is so strong that it resists separation, e.g. by razor blade and/or wet treatment. The use of a pouch structure wherein only the border parts of the plastic sheets are sealed is not sufficiently tamperproof since after cutting around the edge of the original card the pouch can be opened and some information such as the photograph can be removed and replaced by other information before resealing the pouch.
Ideally, to avoid said shortcoming a "security seal" is established between the information-bearing element of the card or document and the plastic. As described in U.S. Pat. No. 4,151,666 the security seal ensures that if one should succeed in the removal of the plastic cover sheet also a substantial portion of the information containing part of the document will also be removed so that a visibly damaged part remains adhering to the support. In this way a protection against mere substitution of information is obtained discouraging alteration of sealed documents.
According to an embodiment described in said U.S. Pat. No. 4,151,666 a tamperproof identification document is made by lamination of transfer prints in envelopes of the type shown in its FIGS. 4 and 5. Each of the envelopes of said Figures has an adhesive system capable of providing a "security seal". The exemplified transfer prints are actually silver complex diffusion transfer photographs which usually contain a portrait of the bearer together with photographic information relating to her or him. Diffusion transfer photographs are extensively used in I.D. card issuance systems known as "instant issuance" systems. In such systems, the intended bearer of the I.D. card is photographed in a card issuance station where appropriate photographic and lamination materials are assembled with a proper lamination equipment.
According to a more recent method described e.g. in U.S. Pat. No. 4,738,949 and published European Patent Application 0 283 048 the "identity information" is reproduced by thermal printing wherein the image is formed by a thermally transferred dye. Thermal dye transfer proceeds according to one common technique by so-called dye sublimation in which a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, applied by a thermal printing head or modulated laser beam, is transferred to a receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of the heat applied to the dye-donor element. The thermal transfer prints, e.g. colour photographs obtain their information e.g. by means of a colour video camera yielding electronic signals that modulate the heating of said thermal printing head or laser beam.
Many of the known methods for producing I.D. cards involve the use of heat-activatable adhesive systems. According to a very convenient embodiment the heat-activatable adhesive system makes part of a pouch or similar assembly structure wherein it is present as an inner layer applied respectively to a support and cover sheet member that are pre-affixed to each other at one edge. The I.D. information sheet, which contains for example a portrait of the intended bearer, is inserted between the inner adhesive layers of the pouch and the lamination is carried out in a platen press, roll laminator or the like to provide an effective security seal for the final product.
The heat-activatable adhesive system is adapted according to the type of I.D. information sheet to be laminated. For example, as described in U.S. Pat. No. 4,101,701 an I.D. card laminar structure includes a transparent polyester front sheet the inner surface of which is provided with a first layer of a heat-activatable adhesive comprising an ethylene acrylic acid copolymer or an ethylene ethyl acrylate copolymer or mixtures of both and a second layer comprising a mixture having a ratio of about 2/1 of polyvinyl alcohol and poly-4-vinylpyridine such for laminating thereto a dye diffusion transfer photograph wherein the image is formed through silver halide photography in a hydrophilic colloid layer comprising a mixture of polyvinyl alcohol and poly-4-vinylpyridine.
In the production of laminates having a hydrophobic information carrier as is the case e.g. with I.D. information sheets wherein the information is printed by thermal dye transfer the image-receiving layer includes as described e.g. in U.S. Pat. No. 4,507,349 polymers selected from the groups of a polycarbonate, polyurethane, polyester, polyamide, polyvinyl chloride, acrylonitrile block copolymer, cellulose triacetate, acrylamide, polyacrylonitrile-co-butadiene-styrene (ABS), and mixtures thereof.
An adhesive resin material that may serve in lamination for firmly bonding the thermal dye transfer image receiving layer to a transparent scratch resistant plastic cover sheet such as a polyethylene terephthalate sheet, is e.g. a polyethylene or atactic polypropylene.
When using an adhesive layer made of such low softening point polymer in combination with an image-receiving layer containing a pattern of sublimable dye heat lamination will give rise to bleeding, i.e. line spreading, of the transferred dye which makes said polymers less suited for use in the production of laminates including sublimable dyes. On the other hand low softening point polyethylene adhesive layers are plasticized in the heat lamination step for a sufficiently long time to allow the escape of trapped air while joining the members to be laminated and thereby artefacts of included air and local wrinkling of the adhesive layer are avoided.
The problem of bleeding has been resolved by the use of adhesive layers that have a higher softening temperature and that flow less than polyethylene, e.g. layers consisting of co-polyesters.
These layers, however, require the use of higher pressures and higher temperatures in order to obtain a satisfactory sealing.
We have found that common heat-laminating apparatus that comprise a pair of inlet rollers, a pair of outlet rollers and two springbiased pressure plates between the rollerpairs that exert a uniform pressure on the sandwich transported between them, unsufficiently evacuate air that is present between the image of the sheet and the sealing film so that a plurality of isolated minuscule opaque islands may be created that make the image worthless.
Attempts to improve the operation of these apparatus by increasing the bias on the pressure plates have failed since it has been shown that the transport of the sandwich between the plates occurs shockwise or becomes even impossible.
Thus, laminating apparatus of a type as disclosed in JP UM 53-30531 are not suited for the sealing of laminates by means of adhesive layers having a higher softening temperature.